The RAK4200 (as well as all other LoRa modules) have FCC certification, but I can’t find any documentation about what antenna they were certified with. Using the same antenna in an end product will greatly simplify the certification process.

Does anyone know where to find what antenna(s) are used with the FCC certification of this module?

HI lancepitka,
As you know RAK4200 just a module not a final product. The shapes of the antenna depends on the final product of the customer .And we have left a margin of 3dBi for the antenna. so your antenna should not exceed 3dBi. And for FCC test we just use a small antenna is about 0.5dBi.